mgcv (version 1.9-1)

inSide: Are points inside boundary?

Description

Assesses whether points are inside a boundary. The boundary must enclose the domain, but may include islands.

Usage

inSide(bnd,x,y)

Value

The function returns a logical array of the same dimension as x and y. TRUE indicates that the corresponding x, y point lies inside the boundary.

Arguments

bnd

This should have two equal length columns with names matching whatever is supplied in x and y. This may contain several sections of boundary separated by NA. Alternatively bnd may be a list, each element of which contains 2 columns named as above. See below for details.

x

x co-ordinates of points to be tested.

y

y co-ordinates of points to be tested.

Author

Simon N. Wood

Details

Segments of boundary are separated by NAs, or are in separate list elements. The boundary co-ordinates are taken to define nodes which are joined by straight line segments in order to create the boundary. Each segment is assumed to define a closed loop, and the last point in a segment will be assumed to be joined to the first. Loops must not intersect (no test is made for this).

The method used is to count how many times a line, in the y-direction from a point, crosses a boundary segment. An odd number of crossings defines an interior point. Hence in geographic applications it would be usual to have an outer boundary loop, possibly with some inner `islands' completely enclosed in the outer loop.

The routine calls compiled C code and operates by an exhaustive search for each point in x, y.

References

https://www.maths.ed.ac.uk/~swood34/

Examples

require(mgcv)
m <- 300;n <- 150
xm <- seq(-1,4,length=m);yn<-seq(-1,1,length=n)
x <- rep(xm,n);y<-rep(yn,rep(m,n))
er <- matrix(fs.test(x,y),m,n)
bnd <- fs.boundary()
in.bnd <- inSide(bnd,x,y)
plot(x,y,col=as.numeric(in.bnd)+1,pch=".")
lines(bnd$x,bnd$y,col=3)
points(x,y,col=as.numeric(in.bnd)+1,pch=".")
## check boundary details ...
plot(x,y,col=as.numeric(in.bnd)+1,pch=".",ylim=c(-1,0),xlim=c(3,3.5))
lines(bnd$x,bnd$y,col=3)
points(x,y,col=as.numeric(in.bnd)+1,pch=".")